1. Field of the Invention
The present invention relates to a membrane for separating a water-organic solvent (hereafter, referred to as "a water-organic solvent separation membrane") and more particularly to a separation membrane for efficiently permeating and separating an organic solvent from a water-organic solvent mixture to purify the water or concentrate and recover the organic solvent.
2. Description of the Prior Art
Utilization of membranes in the step of separation and purification of gases or fluids has recently been increasing. This is not only because the use of membranes makes it possible to carry out the separation with less energy than is required by conventional separation techniques such as distillation but also because it gives an expectation that substances susceptible to thermal denaturation when separated by distillation, azeotropic mixtures or isomers can be subjected to separation.
Examples of utilizing separating membranes currently put into practice include preparation of an ultra-pure water or freshening of seawater using a reverse osmosis membrane, artificial dialysis using an ultrafiltration membrane, application to food industry such as separation of beer yeast, oxygen enrichment using an oxygen-enriching membrane for combustion or medical treatment.
Although membranes for use in dehydration from various water-organic solvent mixtures such as water-alcohol have been put into practical use, those membranes for separating organic solvents from mixtures containing organic solvents in low concentrations have not yet reached into a stage of practical use. However, once the separation of organic solvents from mixtures containing organic solvents in low concentrations is made possible, it is useful for the separation of organic solvents, i.e., substances contained in industrial waste water or underground water and causing environmental pollution, substances harmful to humans, substances which can be recovered and reused, or substances which are objective substances.
For selective permeation or separation of organic solvents from water-organic solvent mixtures, it is sufficient to use membrane materials having affinity higher for organic solvents than water. Separation of water-alcohol or the like has been studied using various membrane materials such as a polydimethylsiloxane membrane, a membrane of a polydimethylsiloxane containing zeolite silica, a polystyrene/polyfluoroalkylate copolymer membrane, and a poly(1-trimethylsilyl-1-propyne) membrane currently considered to have the highest permeability to gases. However, the poly(1-trimethylsilyl-1-propyne) membrane is practically unsatisfactory because it has a separation factor on the order of about 5 to 40. On the other hand, polydimethylsiloxane has a low mechanical strength and therefore is difficult to be formed as a thin film. Thus, no separation membrane has hitherto been obtained that has a high separability for organic solvents.
Examples of the techniques used in the separation of fluids include a pervaporation method. This method is useful as a technique for separating azeotropic mixtures or mixtures of substances having boiling points close to each other that have been heretofore not easily be separated by distillation or the like.
The membrane currently put into practical use in the pervaporation method would be only a polyvinyl alcohol-based composite membrane developed by GFT AG of former West Germany. It is used for the dehydration of azeotropic mixtures such as water-alcohol. However, no membrane for selective separation of organic solvents has been known that has a performance enough to separate an alcohol from an alcohol solution in a low concentration obtained as a result of fermentation or the like.
No membrane has been developed yet that has a performance enough to recover organic solvents which are contained in industrial waste water or underground water. When a distillation method is used for separating organic solvents from stock solutions containing organic solvents having boiling points higher than water in low concentrations, much energy will be required. Also, if adsorption, extraction or the like is utilized, a step of isolating organic solvents will be necessary after the treatment concerned.
Hence, there is a demand for development of a separation membrane for selectively separating organic solvents for use in pervaporation methods, which are advantageous over other separation methods.